1. Technical Field
The present invention relates to a semiconductor laser drive apparatus and an image forming apparatus using the semiconductor laser drive apparatus.
2. Background Art
A semiconductor laser drive apparatus used in a high-speed, high-definition laser printer, digital copier, and the like, needs to drive a semiconductor laser element with a desired light-emission delay and light-emission pulse width. Hence, a technique is known in which a threshold current at which the semiconductor laser element emits light is detected in advance, and the threshold current is caused to flow through the semiconductor laser element as a bias current immediately before the semiconductor laser element emits light, to activate the semiconductor laser element. By this, light-emission delay is reduced, enabling to obtain an excellent light-emission pulse width at turn-on.
In addition, the semiconductor laser drive apparatus has a shading correction function. The term “shading” as used herein refers to variations in density caused by nonuniformity of the amount of light of a laser beam which scans an exposed surface (image surface). To eliminate shading, the semiconductor laser element emits light such that the amount of light is uniform over the exposed surface (shading correction).
A conventional semiconductor laser drive apparatus merely detects only one threshold current from a slope of the amount of light emission with respect to a drive current for when a semiconductor laser element is turned on at a desired amount of light emission. More specifically, when the amount of light emission is represented as a function of the drive current, a threshold current is determined from an intersection of a tangent line at the desired amount of light emission and a straight line of an amount of light emission of 0 mW. Therefore, when the slope of the amount of light emission with respect to the drive current is small, a threshold current to be detected also tends to be small. In the case of using a semiconductor laser element where the linearity of the amount of light emission with respect to the drive current is lost, such as that the slope of the amount of light emission with respect to the drive current gets smaller as the drive current gets larger, a detected threshold current is smaller than the actual threshold current of the semiconductor laser element. In this case, there is a problem that an excellent light-emission delay and light-emission pulse width cannot be obtained.